A measuring method that uses a sensing device equipped with a quartz sensor using a quartz resonator for sensing a substance in minute quantities such as an environmental pollutant e.g. dioxin or the like, or a disease marker for the hepatitis C virus, a C-reactive protein (CRP), or the like has been widely known.
To be more specific, the measurement method is conducted in such a manner that an adsorbing layer is formed in advance on an excitation electrode on one surface side of the quartz resonator, and the presence/absence of the object to be measured or the concentration thereof in a sample solution is measured by applying a property that when the object is adsorbed, the resonance frequency of the quartz piece varies according to the mass of the adsorbed substance. In Patent Document 1, there is a description that for the purpose that the quartz resonator equipped in the quartz sensor used in this measurement method be oscillated in a stable fashion in an immuno-latex solution, it is desirable to have a structure in which only one surface of the quartz resonator comes in contact with a measurement medium.
Such a quartz sensor is usually called a Langevin type quartz resonator. Though not described in Patent Document 1, the fundamental structure of the Langevin type quartz resonator generally has a composition shown in FIG. 11. 10 in the drawing is a round quartz piece, and foil-shaped electrodes 11 and 12 are respectively formed at the center of both surfaces. Supporting line members 13 and 14 to take out an electric signal outside, lead wires of, for instance about 0.5 mm in diameter are connected to these electrodes 11 and 12. A base 16 having a recess 15 is placed on the other surface side of the quartz piece 10. The quartz piece 10 and the base 16 are adhered firmly by an adhesive 17, thereby forming an airtight space enclosed by the quartz piece 10 and the recess 15.
In recent years, further control of toxic substances which have a large effect on an environment such as the above-described dioxin or the like that has been demanded from the viewpoint of environmental protection, and attempts to achieve measurement in the ppt level have been widely conducted. In a quartz resonator, the resonance frequency of the quartz resonator increases as the thickness of the quartz piece decreases. From the Sauerbrey equation, the greater the frequency generated by the quartz resonator, the more the amount of deviation in frequency in regard to the amount of change in mass of the object. In other words, when the quartz piece becomes thinner and thinner, the measurement sensitivity of the quartz sensor increases, so that the measurement of a substance in minute quantities becomes possible. Therefore, reduction in the thickness of the quartz piece is required.
The technology to reduce the layer thickness of a quartz piece has progressed at present, and it has become possible to manufacture a quartz piece with a thickness of several to several tens of μm. When a quartz resonator is composed using this thin layered quartz piece, however, the quartz resonator easily receives the influence of surface tension of the sample solution to be measured, and there is a possibility of not causing oscillation owing to the surface tension or not generating oscillation due to stabilization. When liquid 10B drops down on a plane, it rounds in a convex shape due to the surface tension as shown in FIG. 12A, and in a recess, the liquid surface is curved in a concave shape as shown in FIG. 12B, which generates a stress acting on the plane due to the surface tension. Accordingly, when a quartz resonator 10A is arranged in the plane portion in FIG. 12A or a bottom portion in FIG. 12B, a stress acts on the quartz resonator 10A. Accordingly, in the structure shown in FIG. 11, a stress acts on the quartz resonator 10A in a manner shown by the arrow due to the surface tension of the sample solution. If a housing area for the sample solution is taken widely, the effect of the surface tension can be reduced, but it causes the quartz sensor upsize. Therefore, it is practically inevitable for the quartz resonator 10A to avoid influence of the surface tension of the sample solution, and if reduction in thickness of the quartz piece progresses, the influence of the stress applied on the quartz piece in regard to the deviation in the natural frequency becomes large, and sometimes it would not oscillate anymore.
In Patent Document 2, there is a description of technology of the Langevin type quartz sensor. The technology includes the step of forming a passage space for a sample solution on one surface side of the quartz resonator and supplying the sample solution from an inflow opening to conduct measurement while flowing out the sample solution from an outflow opening. Since, however, the method of flowing the sample solution in this manner causes a large stress on the thin-layered quartz piece, such a structure cannot be adopted at all, and the structure to solve the surface tension issue is not shown.
Patent Document 1: Japanese Patent Application Laid-open No. 2001-83154 (paragraph 0009, paragraph 0019 and FIG. 1)
Patent Document 2: Japanese Patent Application Laid-open No. Hei 11-183479 (paragraph 0024, FIG. 3 and FIG. 10)